The present invention relates generally to a molding apparatus which includes apparatus for coating the inner surface of a mold with a fluidized coating and recovering overspray prior to a molding operation.
Molded parts are readily manufactured from sheet molding compound ("SMC") which is molded into the desired shape within a pressurized mold. Such parts often exhibit poor surface quality (e.g., porosity, waviness, pits, etc.) and typically require special treatments with paint or liquid, in-mold coatings in the event colored surfaces are desired. Accordingly, significant expense may be incurred in improving the surface quality and painting or otherwise coating the surface of the parts.
One approach to eliminating costly post-molding treatments of a molded part is disclosed in U.S. Pat. No. Re 28,068 to Lemelson. This patent discloses an apparatus for selectively depositing a particulate coating against the surface of a mold. When the mold is subsequently filled with a molding material, the coating and the molding material fuse so as to form an integral structure.
U.S. Pat No. 4,205,028 discloses an apparatus for coating the interior surface of a mold with a coating which forms a surface coating on a product molded from sheet molding compound. The coating is an electrostatically charged powdered thermosetting resin which is sprayed onto the heated interior surface of the mold before the mold is filled. The mold is grounded so that the electrical charge on the powdered resin causes the powder to stick to the surface of the mold. The heat of the mold melts the powder and causes it to flow and cure. Sheet molding compound is added to the mold and the mold is heated to cure the SMC resin. The coating and the SMC resin cure so as to form an integral product having a smooth outer surface.
A significant problem associated with the coating of a mold inner surface is that of overspray. The practice of applying an electrostatic charge to the powder as it emerges from a spray gun and grounding the mold surface increases the amount of sprayed powder which adheres to the mold surface. However, even when an electrostatic charge is applied to the powder, very substantial amounts of powder fail to adhere to the mold and are lost as overspray. This overspray must then be collected and reused in order to reduce the cost of the coating process. Whether the powder is reused or not, it must be contained and collected in order to prevent contamination of the surrounding atmosphere. The cost of containing the powder in a spraying room and the cost of collecting the oversprayed powder can be very significant.
U.S. Pat. No. 4,472,451 to Mulder discloses an apparatus for powder coating the interior surface of a heated mold cavity whereby electrostatically charged powder is sprayed onto the surface of the mold which is surrounded by a mask. The mask includes a shield positioned adjacent the shear edge of the mold and vacuum conduits are disposed beneath the shield which engage with vacuum ducts to draw off overspray. The combination of the shield and vacuum conduits are intended to prevent the build-up of the powder on hot surfaces of the mold external of the mold cavity. The overspray recovery apparatus consists of a set of pipes which provide limited evacuation capability and which are susceptible to blockage and associated problems.